Both gear driven and chain driven cam drive arrangements are known in the motorcycle engine art. These cam drive arrangements involve one or more cams, each of which lifts one or more sets of intake and exhaust valves through corresponding lobes positioned on the camshaft. In the known gear drive arrangements, a crankshaft gear directly drives either a sole camshaft gear, or a primary camshaft gear which in turn drives two or more secondary camshaft gears. Known chain drive arrangements are employed in either a push rod configuration or in an "overhead cam" configuration. In the overhead cam chain drive configuration, one or more crankshaft sprockets are connected to respective overhead cam sprockets with one or more long chains. In the push rod chain drive configuration most pertinent here, typically a crankshaft sprocket is connected to a primary camshaft sprocket with a chain; a second chain then connects typically two secondary camshaft sprockets, one of which is on the same shaft as the primary camshaft sprocket.
In 1998 (for model year 1999), Harley-Davidson introduced a new big twin engine called the "TWIN CAM 88." That engine employs a chain drive arrangement generally as depicted in FIG. 1. There are a number of problems associated with the TWIN CAM 88 cam drive design. The design employs chain tensioners pressing against the outside of each chain with the intent that the chain be kept from going slack. However, over time the tensioners wear down, and the chains stretch, both of which contribute to chain slack that results in inaccurate cam timing. Material from the tensioners and chains wearing also contributes to engine debris. Also, the high force (on the order of 50 lbs.) the tensioners apply to the chains is transmitted to the camshafts, which increases camshaft bearing wear. Other shortcomings have become apparent when modifying the engine for improved performance. For example, when high performance valve springs are installed, horsepower is lost due to inadequate performance of the chain tensioners. When cams with larger, high lift cam lobes are installed, as depicted in FIG. 1, the front and rear cam lobes will interfere with one another due to the closeness of the two camshafts and their direction of rotation. The TWIN CAM 88 chain drive arrangement also requires substantial maintenance because the chains and chain tensioners wear. Disassembly and assembly of the arrangement is also time consuming because of, among other things, the various parts that must be correctly assembled together, as for example accurately timing the crankshaft and camshafts. Chain drive arrangements are also inherently dangerous because a chain break can result in catastrophic engine damage or even rider injury.
It can be seen that a method is needed for modifying a chain drive cam arrangement of a motorcycle engine that reduces or eliminates these shortcomings.